This invention relates to a method of preparing, and a plant for the preparation of, an extract from a material by extracting the matter from the material using liquid carbon dioxide as the extracting agent and it has particular application to the extraction of the bittering principles from hops.
U.S. Pat. No. 3,477,856 discloses a general process for the extraction of flavours and flavour containing materials from a wide variety of different organic substances using liquid carbon dioxide as the extracting agent. The material to be extracted and the liquid carbon dioxide are passed through an extraction column in which the matter to be extracted is dissolved in the liquid carbon dioxide and the solution of the extracted matter in the liquid carbon dioxide is then passed to an evaporator in which the carbon dioxide is evaporated. The matter extracted remains in the evaporator and the carbon dioxide vapour is condensed in a further, independent, condenser and then re-cycled into contact with the matter to be extracted.
A considerable amount of work has been carried out to produce extracts of coffee and tea using carbon dioxide as the extracting agent but, in general, this has always involved the use of carbon dioxide as a super critical fluid. In particular, super critical carbon dioxide is used to prepare a de-caffeinated coffee. British Pat. No. 1,346,134 is a typical example of one of the various extraction processes. In this Specification, carbon dioxide which is super critical with respect to both pressure and temperature passes through an extraction column packed with ground coffee beans to extract the coffee oil containing the aroma constituents of the coffee. After all of the coffee oil has been extracted, water is introduced into the system and the completely water saturated super critical carbon dioxide is then used to extract the caffeine from the ground coffee in the extraction column. Finally, the coffee oils are returned to the ground, de-caffeinated coffee. Throughout this entire process, the carbon dioxide is always super critical with respect to either temperature or pressure and, during extraction, the carbon dioxide is super critical with respect to both of these. This Specification also discusses the use of liquid carbon dioxide and includes a comparative example showing that, for coffee, approximately eighty times more carbon dioxide is required to be passed through the coffee when the carbon dioxide is in the liquid state than when it is in the super critical state to obtain the required extraction of the coffee oil.
Another British Pat. No. 1,388,581, discusses the use of super critical carbon dioxide as the extracting agent for hops. This Specification mentions that it is possible to obtain an extract from hops using liquid carbon dioxide but again discusses that the dissolving power of liquid carbon dioxide is much lower than that of super critical carbon dioxide. The Specification describes a system in which carbon dioxide gas is compressed until it is super critical with respect to pressure, subsequently heated so that it is super critical with respect to both pressure and temperature, and then passes through an extraction column packed with hops. The super critical carbon dioxide is then cooled so that it is only super critical with respect to pressure and introduced into a separator in which the lower solubility of the matter extracted from the hops in the carbon dioxide, which is only super critical with respect to pressure, causes the extracted matter to fall out of solution. The carbon dioxide is then re-cycled through the compressor and extraction chamber. The Specification describes the extract as being a paste varying in colour from light to dark green. The conventional solvent extractions strip out not only the bittering principles but the chlorophyll and presumably the super critical carbon dioxide extracts some of the chlorophyll from the hops. It is very difficult to remove the chlorophyll and other matter from the extract, for example to obtain an isomerised extract, without also losing some of the flavour components.
All of the extraction plants using super critical carbon dioxide work at very high pressures and so result in the plant being very expensive.